selftests: mlxsw: Add a TBF selftest

Add a test that runs traffic across a port throttled with TBF. The test
checks that the observed throughput is within +-5% from the installed
shaper.

To allow checking both the software datapath and the offloaded one, make
the test suitable for inclusion from driver-specific wrapper. Introduce
such wrappers for mlxsw.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>